1. Field of the Invention
This invention pertains to the equalization regulation of the currents of several inverters connected in parallel.
2. Discussion of the Background of the Invention and Material Information
The realization or utilization of fast inverters with several individual devices connected directly thereto, in parallel, is possible only With-appreciable expenditures on the part of the electronic control and regulating system. This expenditure is tolerable only when certain demands on the inverter cannot be fulfilled in other ways and/or a parallel connection offers decisive advantages over the conventional solutions with an individual device.
An object of the development of present-day motor constructors and designers is the construction of machines that rotate ever more rapidly with so-called "fast-running drives" requiring operating frequencies of above 500 hertz. With the aimed-at power of several hundred kVa, conventional thyristor inverters or GTO-(Gate Turn Off)-inverters can make these frequencies available, at most, only in the form of a fundamental frequency keying.
The disadvantages of this coarse rectangular voltage modulation and the non-sinusoidal current curve resulting therefrom is known sufficiently in the form of additional losses and torque Shocks. Methods have therefore been used for a long time, which subdivide these voltage blocks into small units in time and thus achieve an almost sinusoidal course of motor current. For the inverter, this means that the power switches must be switched on and off correspondingly more rapidly. The faster the actuation of the switches, the finer is the subdivision of the voltage blocks and the more sinusoidal becomes the motor current.
If the operating frequency is now raised, from the usual 50 to 120 hertz to more than 500 hertz, in fast-running drives, the switching frequency of the switches must also be raised correspondingly. This means that the thyristor switches and GTO-switches even more closely approach the limit of their switching speed.
Fast inverters with switching frequencies up to 100 kilohertz, which make a sinusoidal current curve possible even at a starting frequency of 500 hertz, are commercially available only up to a power class of about 50 kVa. A power increase to several hundred kVa with a constantly or consistently high pulse rate is, to the present day, hardly feasible in a conventional manner, hence the reason for parallel connected inverters. In an electrical coupling of the energy delivered by the individual inverters, the individual pairs of the partial converters are connected, one with the other, and the load is connected as usual. The partial inverters can be connected with each other either by way of large connecting chokes, which exercise only a system-caused function, or by way of small chokes, which have only protective functions. The latter case is referred to as direct coupling.
It is known to provide a superordinate total current regulation which adjusts the sum current I.sub.i, of all individual partial inverters, to follow a target magnitude I.sub.s. For this purpose, the currents of all-partial inverters are measured and added. In principle, it does not matter for the parallel connection of the partial inverters whether a PWM, an on-off regulation or another method is utilized in the case of the regulating methods of the superordinate total current regulation.
This total current regulation thus makes the actual value follow the target value in that it decides whether all partial inverters together will supply more or less current in the future, i.e., whether the corresponding "upper" or "lower" power switches in the individual partial inverters will be switched on or off, each time. In other words, the partial inverters are synchronized in that the switching states of their power switches are determined directly by the total current regulation.
The division of the total current, over the individual partial inverters, takes place according to a preset division rule which will usually provide the same currents for all partial inverters.
Since individual partial inverters can, however, now have very different properties (for example different internal resistances or internal inductances in consequence of component tolerances, production tolerances, different environmental conditions and so forth), deviations often result from this current division rule. For this reason, a subordinate equalizing regulation is utilized, which makes certain of the prescribed current division between the individual partial, inverters.
An example of such an equalizing regulation is described in prior art German Patent Publication DE 4 023 207 C1. Therein, the currents of the individual partial inverters are regulated in such a manner that the instant of switching-on and switching-off of each individual inverter is regulated on the basis of the difference between the current delivered by it and the current value corresponding to its target portion of the total current. Thereby, the currents of the individual partial inverters are made to follow their common mean value and a wide divergence of the Currents is thus prevented. It is, however, not the deviation from the mean value of all currents that is decisive for a current division contrary to the rule, but, for example, in the case of a direct current divider, the greater negative deviation (minimum value) when the power switches are deactivated and the greatest positive deviation (maximum value) when the power switches are activated.